Time-of-flight mass spectrometers are widely used to determine the mass to charge ratio of charged particles on the basis of their flight time along a path. The charged particles, usually ions, are emitted from a pulsed source in the form of a packet, and are directed along a prescribed flight path through an evacuated space to impinge upon or pass through a detector. (Herein ions will be used as an example of charged particles.) In its simplest form, the path follows a straight line and in this case ions leaving the source with a constant kinetic energy reach the detector after a time which depends upon their mass to charge ratio, more massive ions being slower. The difference in flight times between ions of different mass-to-charge ratio depends upon the length of the flight path, amongst other things; longer flight paths increasing the time difference, which leads to an increase in mass resolution. When high mass resolution is required it is therefore desirable to increase the flight path length. However, increases in a simple linear path length lead to an enlarged instrument size, increasing manufacturing cost and require more laboratory space to house the instrument.
Various solutions have been proposed to increase the path length whilst maintaining a practical instrument size, by utilising more complex flight paths. Many examples of charged particle mirrors or reflectors have been described, as have electric and magnetic sectors, some examples of which are given by H. Wollnik and M. Przewloka in the Journal of Mass Spectrometry and Ion Processes, 96 (1990) 267-274, and G. Weiss in U.S. Pat. No. 6,828,553. In some cases two opposing reflectors or mirrors direct charged particles repeatedly back and forth between the reflectors or mirrors; offset reflectors or mirrors cause ions to follow a folded path; sectors direct ions around in a ring or a figure of “8” racetrack. Herein the terms reflector and mirror are used interchangeably and both refer to ion mirrors or ion reflectors unless otherwise stated. Many such configurations have been studied and will be known to those skilled in the art.
Electrostatic trapping is also well known and a class of traps utilise orbital trapping. Orbital electrostatic trapping was demonstrated by K. H. Kingdon (Phys. Rev. 21 (1923) 408) in a trap comprising an outer electrode structure and an inner electrode structure, the outer structure surrounding the inner. Ions orbit about the inner electrode structure in the region between the inner and outer electrode structures.
A type of orbital electrostatic trap utilising opposing linear fields which result in harmonic ion oscillations in the direction of an analyser axis is used in the Orbitrap™ mass analyser, of A. A. Makarov (U.S. Pat. No. 5,886,346 and Anal. Chem. 72 (2000) 1156). A single spindle-like inner electrode structure is surrounded by an outer electrode structure of barrel-like form.
C. Köster (Int. J. Mass Spectrom. Volume 287, Issues 1-3, pages 114-118 (2009)) describes harmonic ion trapping in structures comprising a plurality of inner electrodes all surrounded by an outer electrode structure.
However these prior art electrostatic traps in which ions orbit around inner electrodes and/or the analyser axis as so described have not been used to function as time of flight mass spectrometers as ions spread out around the inner electrode(s) with ions of the same mass to charge ratio forming rings. Ejection of such rings to a detector cannot be accomplished easily without disrupting other rings of ions within the trap and means to sequentially eject ions of increasing or decreasing mass to charge ratio so as to produce a spectrum were not provided.
Patent SU1716922 describes a two-reflection TOF analyser comprising opposing mirrors elongated along an analyser axis. The mirrors comprise concentric cylinders and ion motion in a direction parallel to the analyser axis is not harmonic. Ions enter the analyser through an aperture set inside the diameter of an outer cylindrical electrode and follow a helical trajectory of constant radius about an inner cylindrical electrode before emerging from an exit aperture and impinging upon a detector. In this apparatus the entrance aperture is set into the analyser structure at the radius at which ions are to circulate. The same or a further aperture is also set into the analyser structure at the radius at which ions are to circulate to enable ions to leave the analyser. The presence of the inset apertures would otherwise distort the field within the analyser and to prevent this, field correction electrodes must be incorporated into the analyser. As described, these introduced obstacles on the path of the ions and the fringe field correction was not perfect, resulting in a reduction in sensitivity and resolution of the spectrometer. Most importantly, the presence of fringe field correction electrodes limited the number of oscillations to just one full oscillation (one back and one forward pass).
Against this background, the present invention has been made.
A brief glossary of terms used herein for the invention is provided below for convenience; a fuller explanation of the terms is provided at relevant places elsewhere in the description.
Analyser electrical field (also termed herein analyser field): The electric field within the analyser volume between the inner and outer field-defining electrode systems of the mirrors, which is created by the application of potentials to the field-defining electrode systems. The main analyser field is the analyser field in which the charged particles move along one or more main flight paths.
Analyser volume: The volume between the inner and outer field-defining electrode systems of the two mirrors. The analyser volume does not extend to any volume within the inner field-defining electrode system, nor to any volume outside the inner surface of the outer field-defining electrode system.
Angle of orbital motion: The angle subtended in the arcuate direction as the orbit progresses.
Arcuate direction: The angular direction around the longitudinal analyser axis z. FIG. 1 shows the respective directions of the analyser axis z, the radial direction r and the arcuate direction ø, which thus can be seen as cylindrical coordinates.
Arcuate focusing: Focusing of the charged particles in the arcuate direction so as to constrain their divergence in that direction.
Asymmetric mirrors: Opposing mirrors that differ either in their physical characteristics (size and/or shape for example) or in their electrical characteristics or both so as to produce asymmetric opposing electrical fields.
Beam: The train of charged particles or packets of charged particles some or all of which are to be separated.
Belt electrode assembly: A belt-shaped electrode assembly extending at least partially around the analyser axis z.
Charged particle accelerator: Any device that changes either the velocity of the charged particles, or their total kinetic energy either increasing it or decreasing it.
Charged particle deflectors: Any device that deflects the beam.
Detector: All components required to produce a measurable signal from an incoming charged particle beam.
Ejector: One or more components for ejecting the charged particles from the main flight path and optionally out of the analyser volume.
Entry port: portal through which ions pass on joining a main flight path. The portal may be within the analyser volume or at the boundary of the analyser volume.
Equator, or equatorial position of the analyser: The mid-point between the two mirrors along the analyser axis z, i.e. the point of minimum absolute electrical field strength in the direction of the analyser axis z within the analyser volume.
Exit port: portal through which ions pass on leaving a main flight path as they proceed to leave the analyser volume. The portal may be within the analyser volume or at the boundary of the analyser volume.
Field-defining electrode systems: Electrodes that, when electrically biased, generate, or contribute to the generation of, or inhibit distortion of the analyser field within the analyser volume.
Injector: One or more components for injecting the charged particles onto the main flight path through the analyser.
Main flight path: The stable trajectory that is followed by the charged particles for the majority of the time that the particles are being separated. The main flight path is followed predominantly under the influence of the main analyser field. There may be a plurality of main flight paths.
m/z: Mass to charge ratio
Receiver: Any charged particle device that forms all or part of a detector or device for further processing of the charged particles.